In all, I agree with billing at a daily rate. Personally I bill in half day increments for minor maintenance with smaller loyal clients and daily or even weekly for significant slices of work. A worthwhile client won't pay much attention to how things break down in smaller increments provided they know up front that there is a minimum block.

One of the healthy side effects is that you'll rarely end up with the "can you just quickly add x" requests, which can be far more time consuming.